The reason why you had no answer so far to your question is because you posted it to the wrong place. See Getting the most of the community when you have a Marketo question
Normally by default, Marketo 2 column forms move to 1 column automatically on mobile. You do not need to manage it.
If it does not happen, you should consider to simplify your custom css as it is probably preventing it to do so.
What you can also do is to set your CSS with media queries so that some of your CSS instructions apply to desktop only.